Polymerization process and product thereof



April 28, 1970 w. D. NIEGISCH ETA!- 3,509,075

POLYMERIZATION PROCESS AND PRODUCT THEREOF Filed May 4, 1966 INVBJTDRSWALTER D.NIEGISCH WILLIAM E. LOEB United States Patent 3,509,075POLYMERIZATION PROCESS AND PRODUCT THEREOF Walter D. Niegisch, Watchung,and William E. Loeb, Martinsville, N.J., assignors to Union CarbideCorporation,

a corporation of New York Filed May 4, 1966, Ser. No. 548,821 Int. Cl.C08g 33/00 US. Cl. 2602 9 Claims ABSTRACT OF THE DISCLOSURE A processfor preparing highly extensible poly-p-xylylenes at high depositionrates through the condensation of p-xylylene diradicals at pressuresbelow 75 mm. Hg and at temperatures between about 0 C. and 196 C.

This invention relates to a process for the preparation of p-xylylenepolymers exhibiting prolonged high extensibility and to a crystallinepolymorph produced thereby.

It is known that various poly-p-xylylenes can be prepared by a pyrolyticpolymerization of p-xylene and substituted derivatives thereof. Thisprocess, first disclosed by M. Szwarc, Disc. Faraday Society, 2 46(1947), termed the Szwarc process, basically consists of a hightemperature pyrolysis (800-1000 C. at subatmospheric pressures) of thestarting p-xylene followed by cooling the pyrolysis vapors to apolymerization temperature, such as by condensing the vapors on asubstrate at about room temperature. Upon cooling and condensation, thereactive diradical formed in the pyrolysis instantly polymerizes andforms a polymeric film on the substrate. However, the high operatingtemperature of this process and the exceptionally low yield of polymer(about 1012 percent of theoretical) left much to be desired forcommercial applications.

For instance, in the Szwarc process, operating temperatures of 800C.1000 C. were found to cleave off hydrogen or other substituent groupsin the para-xylene because of the instability of such substituent groupsat such temperatures resulting in the formation of crosslinked polymers.Thus, such a procedure is unsuitable for the preparation of linearpara-xylylene polymers.

In addition, these high operating temperatures, even with unsubstitutedpara-xylene, were found to char the polymer to such an extent thatoff-color, undesirable polymers resulted. With substituted polymers,charring became so severe that it could not be tolerated.

Thus, the polymer of the Szwarc process has been found to be of suchnon-uniform quality and so generally crosslinked and insoluble inlow-boiling solvents as to limit its use even when of acceptablequality. Such polymers are generally only soluble with difliculty incertain few high boiling solvents.

Kaufman, Mark & Mesrobian (J. Pol. Sci., 13 (1954)), investigating thepolymer, concluded that the polymer was extensively cross-linked and wasnot the linear polymer that Szwarc presumed it to be. They also observedthat after 48 hours, the Szwarc polymer was brittle at room temperaturealthough it could be stretched 250-400 percent at temperatures above 150C. Hence, the Szwarc polymer is further limited in commercialapplication since the polymer embrittles after a short period of timeand does not exhibit useful extensibility.

W. F. Gorham in copending application Ser. No.

Patented Apr. 28, 1970 528,413 filed Feb. 18, 1966, now US. Patent No.3,342,- 754, granted Sept. 19, 1967, describes the preparation of trulylinear p-xylylene polymers produced in nearly quantitative yields byheating a cyclo di-p-xylylene having up to about 6 aromatic nuclearsubstituent groups to a temperature between about 450 C. and 700 C. fora time suflicient to cleave substantially all of the di-pxylylene intovaporous p-xylylene diradicals but insufficient to further degrade thesaid diradicals and at a pressure such that the partial pressure of thevaporous p-xylylene diradicals is below 1.0 mm. Hg and preferably below0.75 mm. Hg, and cooling the vaporous diradicals to a temperature below200 C. and below the ceiling condensation temperature of the p-xylylenediradical species present in the pyrolysis vapors. Condensation of thediradicals yields tough, linear, non-fluorescent pxylylene polymers. Allknown p-xylylene diradical species have condensation temperatures belowabout 200 C. and above or about room temperature. Typically, theextensibility of the Gorham polymers at room temperature has been foundto be about 1015 percent.

It is an object of the present invention to provide a process wherebylinear p-xylylene polymers are produced exhibiting high extensibilitiesenabling their use as an encapsulant capable of compensating for thethermal expansion or change of state of an encapsulated material.

It is another object of the present invention to provide a processwhereby linear p-xylylene polymers are produced at high growth ratesexhibiting increased tensile strength, elongation at break and yieldelongation.

It is still another object of the present invention to provide a novel,crystalline polymorph of poly-p-xylylene.

These and other objects are accomplished by the present invention whichprovides, in one aspect thereof, a process for the preparation ofpoly-p-xylylene exhibiting high extensibility which comprises pyrolyzingcyclic di-pxylylene having the structural formula:

at a temperature between about 450 C. and 700 C. for a time sufficientto cleave substantially all of the di-pxylylene into p-xylylenediradicals but insufficient to degrade said diradicals and at a pressuresuch that the partial pressure of the p-xylylene diradicals is belowabout 0.75 mm. Hg, and cooling the diradicals to a temperature betweenabout 0 C. and 196 C. thereby condensing said diradicals and forming ahighly extensible poly-pxylylene.

In another aspect of the present invention, the poly-pxylylene producedby the process of the present invention contains a heretofore unknownpolymorph referred to herein as the gamma modification ofpoly-pxylylene.

In order to provide a more comprehensive understanding of the presentinvention, typical X-ray diffraction patterns of the various polymorphsof poly-pxylylene are shown in the accompanying drawing where- 1n:

FIGURE 1 presents the X-ray diffraction pattern of the novel gamma (7)polymorph of poly-p-xylylene;

FIGURE 2 presents the X-ray diffraction pattern of the known alpha (a)polymorph of poly-pexylylene; and

moQ-cm Pyrolyticcleavage of the cylic dimer results in the splitting ofthe dimer into two reactive diradicals, each of which are represented bythe basic structure:

The reactive diradicals are prepared by pyrolyzing di-para-xylylene at atemperature less than about 700 C., and preferably at a temperaturebetween about 550 C. to about 600 C. At such temperatures, essentiallyquantitative yields of the reactive diradical are secured. Pyrolysis ofthe starting di-p-xylylene begins at about 450 C. regardless of thepressure employed. Operation in the range of 450550 C. serves only toincrease time of reaction and lessen the yield of polymer secured. Attemperatures above about 700 C., cleavage of the substituent group canoccur, resulting in a trior poly-functional species causingcross-linking of highly branched polymers.

Pyrolysis temperature is nearly independent of the system operatingpressure. It is, however, preferred that reduced or subatmosphericsystem pressures be employed. For most operations, system pressurewithin the range of 0.0001 to mm. Hg is most practical. However, ifdesired, greater pressures can be employed by using inert vaporousdiluents such as nitrogen, argon, carbon dioxide, steam and the likewhich can either vary the optimum temperature of operation or change thetotal effective pressure in the system. In fact, essentiallyquantitative yields of clear, tough linear poly-p-xylylene is secured atsystem pressures up to atmospheric as long as the diradical partialpressure is maintained below 1.0 mm. pressure.

Heretofore, p-xylylene diradicals have been polymerized by maintainingthe polymerization zone at a temperature below the ceiling condensationtemperatures of the diradical species involved. All observed ceilingcondensation temperatures have been between about room temperature,i.e., C. and 200 C. but vary to some degree upon the operating pressureinvolved. For example, at 0.5 mm. Hg pressure, the followingcondensation and polymerization ceilings have been observed for thefollowing diradicals:

C. p-Xylylene 25- Chloro-p-xylylene 70- 80 n-Butyl-p-xylylene 130-140Carbomethoxy-p-xylylene 130-140 Dichloro-p-xylylene 130-140 Thus,p-xylylene polymers have been made by cooling the vaporous diradicals toa temperature at or just below the condensation temperature of theparticular diradical species involved. P-xylylene polymers formed inthis manner have been found to exhibit extensibilities at roomtemperature of from about 10-15 percent.

It has now been found that cooling the vaporous diradicals, regardlessof ceiling condensation temperature considerations, to temperatures offrom 0 C. to about 196 C. results in p-xylylene polymers exhibitingunexpectedly high extensibilities at room temperature. In general, thep-xylylene polymers produced by the present invention have exhibitedextensibilities in excess of about 100 percent which are substantiallyunaffected by aging of the polymer. Preferably, the vaporous diradicalsare cooled to temperatures of from about -20 C. to about C. Attemperatures above about 0 C., the extensibility of the polymer rapidlydiminishes to that heretofore obtained, i.e., 1015 percent. Attemperatures below about 196 C., coherent films can be deposited but thedeposition rate is too slow to be considered practical.

Polymer quality is dependent on diradical partial pressure in thecondensation zone. Deposition at or above 1.0 mm. partial pressure hasbeen found to yield yellow, highly fluorescent polymers with impairedphysical properties containing stilbene moieties and/ or substantialcrosslinking. As the partial pressure is reduced below 1.0 mm., polymerquality as measured by color, transparency and fluorescence isremarkably improved. At a pressure of 0.75 mm. the polymer is free offluorescence and acceptable in quality although slightly yellow whereasat a pressure of 0.5 mm. or less the quality is excellent with no coloror fluorescence, and is strong and flexible.

Unexpectedly, it has now been found that there is a partial pressurerange within which the p-xylylene polymers attain optimum extensibility.Thus, when the p-xylylene diradicals are condensed and polymerized attemperatures between about 0 C. and 196 C. and at p-xylylene partialpressure up to about 300 microns, optimum extensibility is attained. Inthis manner, poly-pxylylenes have been obtained exhibitingextensibilities, i.e., ultimate elongations, as high as 300 percent. Atp-xylylene partial pressures below about 20 microns, the deposition rateis too slow to economically obtain thick films; however, even, when thinfilms are desired, elongation is apparently reduced. At p-xylylenepartial pressures above about 300 microns, the extensibility of theresulting polymer rapidly falls below about percent. Preferably,p-xylylene partial pressure of from about 200 to 300 microns isemployed.

Formation of poly-p-xylylene is greatly enhanced by operation inaccordance with the preferred aspects of the present invention. It hasbeen found that the growth rate of poly-p-xylylene, i.e., the increasein film thickness per minute, varies directly with the square of thep-xylylene partial pressure, up to pressures of about 300 microns.Additionally, at constant pressure, the growth rate increases withdecreasing substrate temperature. Thus, by depositing p-xylylenediradicals at temperatures below 0 C. and preferably at from about -20C. to about 50 C. and at p-xylylene partial pressure up to about 300microns, up to about 40-fold increases in growth rate as compared toprior deposition methods have been obtained.

Because of the pressure sensitivity involved, common U-tube mercurymanometers, which are virtually impossible to read with accuracy below1.0 mm. are recommended only for indicating system pressure. Even thoughthe diradical is a condensible gas, thermocouple gauges for measuringthe partial pressures can be used and are recommended, if heated toprevent deposition of polymer on the filaments. Preferably, though notalways necessary, the heated thermocouple gauge can be calibratedagainst a McCleod gauge to relate the true partial pressure of thep-xylene diradicals.

It has also been found that annealing the resulting polymers in vacuumat temperatures of from about 100 C. to 300 C. results in a surprisingincrease in yield elongation as opposed to the generally expecteddecrease in yield elongation due to an increase in crystallinity withannealing. The unannealed polymers exhibit a yield elongation of about2-3 percent; whereas the annealed polymers exhibit yield elongation ofabout 10 percent. Thus, through the present invention, especially toughpoly-pxylylene exhibiting increased tensile strength, ultimateelongation and yield elongation are obtained by depositing thep-xylylene diradicals at temperatures of from about 0 to 196 C. atp-xylylene partial pressures below about 300 microns followed by avacuum annealing treatment. The annealing treatment, however, is notconsidered necessary in obtaining the highly extensible polymers of thepresent invention and can be omitted, if desired.

In another aspect of the present invention, a novel polymorph ofpoly-p-xylylene has been found to exist when p-xylylene diradicals aredeposited at temperatures below about 20 C. Poly-p-xylylene having therepeatis unique within the p-xylylene polymer family in that it exhibitspolymorphism. Two polymorphs have heretofore been known to exist. Thealpha modification is metastable and transforms to the stable beta (6)modification at temperatures above 220 C. An additional transition hasnow been observed at 270 C., which is reversible. On the basis of X-rayand optical evidence, the 270 C. transition is considered to be asmectic mesomorph of the beta modification, similar to the analogoustransition of poly(tetrafluoroethylene) at 29 C.

It has now been found that at temperatures below about 20 C., a newpolymorph of poly-p-xylylene referred to herein as the gamma (7)modification is formed. At temperatures between about -20 C. and 50 C.,the alpha (or) and gamma (1 modifications'are codeposited. The relativeamounts formed are dependent upon the temperature with slower depositionrates favoring the formation of the more stable alpha modification. Thegamma (7) modification is the only polymorph formed at substratedeposition temperatures below 50 C. The gamma modification is metastableand is transformed to the alpha modification at temperatures above about80 C. Lattice constants have not been determined for the gamma polymorphdue to the somewhat diffuse nature of the reflections and the fact thatfiber diagrams of uniaxially oriented 'y poly-p-xylylene correspond tothe amodification after orientation. The gamma modification can,however, be identified and distinguished from the alpha and betapolymorphs by a comparison of the interplanar spacings determined fromthe X-ray powder diffraction patterns for the respective polymorphs.Reference is made to the drawing wherein FIGURE 1 represents the X-raypowder diffraction pattern of the gamma (7) polymorph. In comparisonFIGURE 2 represents the X-ray powder diffraction pattern of the alpha(a) polymorph and FIGURE 3, the X-ray powder diffraction pattern of thebeta (5) polymorph. In Table 1, below, the interplanar spacing and therelative intensity of the X- ray powder diffraction patterns of therespective polymorphs are compared. The data was obtained with a 114.6mm. Philips powder camera; nickel filtered copper K, radiation; KV/ma=40/ 20.

TABLE I I=Ralative intensity; d=lnterplanar spacing in Angstrom units;S=Strong; M=Medium2 W=Weakz V=Very.

The p-xylylene polymers of the present invention are readily recoveredfrom the condensation polymerization zone by any convenient means,depending upon the particular zone employed. Where a surface maintainedat temperaturesof from 0 C. to about -196 C. such as a condenser ortrapping device is employed as the polymerization zone, the polymer canbe removed from the wall of the zone by mechanically stripping it off orother similar means. It is not to be implied that the polymers of thisinvention have to be removed or recovered from the depositing surfacesince the most practical of all applications is to have the surface orsubstrate to be coated and protected within or as a part of thepolymerization zone. Small articles can be protected or encapsulatedwith these polymers or planar or irregular substrates of any sort can becoated, with or without masking, for securing the insulative andprotective properties of the poly-pxylylene of this invention.Deposition of the polymer on continuously moving surfaces of paper,metal foils, fabrics and the like can readily be accomplished within thedeposition zone by appropriate design. In particular, the

highly extensible p-xylylene polymers of the present invention areparticularly useful where the encapsulated substrate undergoes changesin density or shape as its environment changes. For example, ice wasencapsulated with highly extensible poly-p-xylylene by condensing andpolymerizing p-xylylene diradicals in a tumbling deposition zonemaintained at temperatures below 0 C. The ice particles were maintainedin constant motion thereby continually exposing fresh surfaces to becoated. After encapsulation, the ice particles were allowed to Warm toroom temperature and melt. The high extensibility of the poly-p-xylyleneencapsulant enabled the encapsulation to closely conform to the changein the shape of the substrate occasioned by its transformation fromsolid ice to liquid water.

The following examples are illustrative of the present invention andshould not be interpreted as a limitation thereof. Unless otherwisenoted, all parts and percentages are by weight.

EXAMPLES 1-9 Cyclic di-p-xylylene prepared in the manner described byPollart in U.S. 3,149,175, was placed within a borosilicate glasssublimation chamber measuring 2 inches in diameter and 4 inches long. Athermocouple gauge registered the pressure at one end of the chamber,the other end of said chamber was connected by a standard taper joint toa 1% inch diameter quartz pyrolysis tube 26 inches long. The cyclicdimer was sublimed at an outside temperature of C. and a system pressureof about 0.2 mm. Hg. The vapors passed through a 6 inch section of thepyrolysis tube (vaporization zone) heated to 200 C. and then a 19 inchlength (pyrolysis zone) maintained at temperatures between about 550C.600 C. The polymerization chamber was connected to the terminalportion of the pyrolysis tube and was maintained at various temperaturesbelow 0 C. by immersion of the polymerization chamber in a DryIce-acetone mixture contained in a Dewar flask. In order to prevent theDry Ice from directly contacting the polymerization chamber, an aluminumshield was placed in the Dewar flask dividing it into two annularcompartments, one containing the polymerization chamber immersed inacetone and the other containing the Dry Ice-acetone mixture. Dry Icewas added periodically to the outer zone to maintain a constanttemperature in the inner Zone which directly cooled the polymerizationchamber. The polymer was recovered by stripping it from the walls of thepolymerization chamber.

In examples wherein the polymer was annealed, the polymerization chamberwas heated to the annealing temperature and the deposited film wasmaintained at the annealing temperature, under vacuum for from about 3to 6 hours and thereafter allowed to cool to room temperature.

Control samples were prepared in the same manner as described aboveexcept that the polymerization chamber was maintained at roomtemperature.

The room temperature tensile properties of poly-p- 1. Process for thepreparation of poly-p-xylylene exxylylene prepared in accordance withthe present invenhibiting high extensibility which comprises pyrolyzingtion are shown in Table II.

TABLE II Substrate p-Xylylene Tensile Elongation Tensile Temperapartialpres- Vacuum annealing Strength at break modulus Example ture C.) Sure(microns) conditions (p.s.i.) (Percent) (p.s.i.)

It can readily be seen that when the polymerization 20 cyclicdi-p-xylylene having the structural formula: temperature is maintainedbelow C., a marked increase in elongation at break is obtained. Also, asthe extremities of the preferred p-xylylene partial pressure range areapproached, a rapid decline in elongation can H C 7 CH2 be observed. 25

EXAMPLES 10-24 I I In the same manner as described in Examples 1-9,dip-xylylene was pyrolyzed and deposited at temperatures below 0 C. toform poly-p-xylylene exhibiting increased extensibility. In Table IIIbelow, the effect of annealing 30 on the yield elongation isdemonstrated. It can be seen at temperature between flbOllt C- a d 700C. for that annealing of the control samples (prepared by dea time Sl1fli 51ent to sulistantlany all of the "P- position at room temperature)had no appreciable effect Y Y Q pfy y dlradlcals but i fficient to deonyield elongation; whereas, annealing of the polygrade said diradicalsand at a pressure such that the parxylylene films prepared by lowtemperature deposition 35 tial pressure of the p-xylylene diradicals isbelow about resulted in yield elongation as high as percent. 0.75 mm.Hg, and cooling the diradicals to a temperature TABLE III YieldElongation, Ultimate Elongation, Polymerization ModulusXlO-fl p.s,i.Yield Strength, p.s.i. Tensile Strength, psi. Percent Percent AnnealTemp./ Temp., 0. RT 2 150 250 RT 150 250 RT 150 250 RT 150 250 RT 150250 -/RI 20/i50 20/250 20/RT 20/150 -20/250 -/RT -25/150 25/250 -IRTs5/150 as/250 /RT 40/150 -40/250 Control +25/RT Do +25/150 Do +25/250 1150 C.=6 hrs, 250 C.=3 hr. anneal.

RT=Room Temperatnre=25 0.

EXAMPLE 25 between about 0 C. and '50 C. thereby condensing saiddiradicals and forming a highly extensible poly-pxylylene.

2. Process as defined in claim 1. wherein the diradicals are cooled andcondensed at temperatures between about 2.0 C. to about C.

3. Process as defined in claim 1 wherein the p-xylylene partial pressureranges up to about 300 microns.

4. Process as defined in claim 3 wherein the p-xylylene partial pressureis from about 200 to 300 microns.

In the same manner described in Examples l-9, di-pxylylene was pyrolyzedand polymerized by condensation in a polymerization chamber immersed inliquid nitrogen at a temperature of 192 C. The poly-p-xylylene film wasdeposited at the rate of 200- angstroms per minute. The resulting filmwas coherent and contained a significant amount er crystallinity.Transmission electron-diffraction pattern studies of the polymerdeposited at -l92 C. clearly establishes that at this temperature, onlythe gamma modification is formed. Annealing the film at 190 c. Processas defined in claim 1 wherein the resulting for several hours caused atransformation to the alpha l 'P' Vacuum annealed at temperatures of (a)modification. Annealing at 265 C. efifects transforfIOm about m 'mationto the beta (,8) modification. 6. Process for the preparation ofpoly-p-xylylene at What is claimed is: high growth rates exhibiting highextensibility which comprises pyrolyzing cyclic di-p-Xylylene having thestructural at a temperature between about 450 C. and 700 C. for a timesufilcient to cleave substantially all of the dipxylylene intop-xylylene diradicals but insuficient to de- 10 grade said diradicalsand at a pressure such that the partial pressure of the p-Xylylenediradicals ranges up to about 300 microns, and cooling the diradicals toa temperature between about 0 C. and 50 C. thereby rapidly condensingsaid diradicals and forming a highly extensible poly-p-xylylene.

7. Process as defined in claim 6 wherein the diradicals are cooled andcondensed at temperatures between about 20 C. to about 50 C.

8. Process as defined in claim 6 wherein the p-Xylylene partial pressureis from about 200 to 300 microns.

9. Process as defined in claim 6 wherein the resulting poly-p-Xylyleneis vacuum annealed at temperatures of from about 100 C. to 300 C.

References Cited UNITED STATES PATENTS 2,999,820 9/1961 Young 260-23,084,146 4/1963 Errede 2602 3,342,754 9/1967 Gorham 2602 OTHERREFERENCES Kaufman et al., Jour. of Polymer Science, vol. XIII (1954),pp. 3-20, pp. 3-11 only needed.

Auspos et al., Journal of Polymer Science, vol. XV

SAMUEL H. BLECH, Primary Examiner US. Cl. X.R.

